Synthetic image devices are today used for many different purposes due to their properties of providing eye-catching optical effects. Besides pure esthetical use, synthetic image devices are also frequently used as security label means in e.g. valuable articles, credit cards, identification documents, bank notes etc. In many applications, the synthetic image devices are merged into another material, resulting in a composite product.
Synthetic image devices are based on the interaction between an array of focusing elements and micro image portions. In some prior art “synthetic images” are also referred to as “integral images”, since the experienced image is composed of a number of parts interpreted together as an integral unit. The relative geometrical relation gives rise to different optical effects, from different levels of 3D to images that move or change its appearance e.g. dependent on the viewing angle. The focusing elements and the micro image portions are typically provided by printing or embossing on/in an essentially transparent polymer film. To this end, different types of printing/embossing plates are typically used. The synthetic image devices are in prior art sometimes divided into two sub-categories; synthetic integral image devices and moiré magnifiers. In moiré magnifiers, the micro image portion corresponds to scaled-down copies of the intended macro image, provided in a regular array with a period that differs from the period of the focusing elements by only a small fraction. In synthetic integral image devices, each focusing element corresponds to an associated cell of micro images, which typically are designed individually for giving rise to the composite image when viewed through the focusing elements. In other words, a moiré magnifier has a regular array of cells with identical content, but with a different periodicity compared to the array of focusing elements. A synthetic integral image device has a regular array of cells with the same periodicity as of the array of focusing elements, but with differing portions within the cells.
However, adopting the synthetic image device view on micro image definition and applying it to the moiré magnifier results in the understanding that the moiré magnifier is actually a special case of the synthetic image device or integral image device. Using the same period as the focusing elements, the micro image plane can be divided into an array of slightly changing micro images, all showing different parts of a repetitive image pattern sometimes containing parts of more than one image icon.
The synthetic images devices can be produced in sheets, printed and/or embossed one by one using printing/embossing plates. For mass production of synthetic image devices, however, a continuous manufacturing is to prefer. By providing a cylindrical plate, a continuous printing/embossing production can be provided. One example of such a manufacturing method is disclosed in the published international patent application WO 2009/085004 A1.
The cylindrical tools used for the printing/embossing can be provided in different ways. One approach is to produce the plates in a flat form. The useful plate area is cut out and the plate is bent in an essentially cylindrical form by positioning two opposite edges against each other. The edges are then welded together, thereby forming a cylindrical tool. The cylindrical tool is then provided onto a roll and put into a printing/embossing device. One example of providing a cylindrical tool according to such an approach is disclosed in the U.S. Pat. No. 5,499,580.
One problem with such an approach is, however, that the weld seam will be more or less visible in the final product. Such a print or embossment from the weld seam region will appear repeatedly along the final product. For smaller synthetic image devices, areas between such weld seam regions can be cut out and used, and the weld seam regions will be disposed. However, in applications where the synthetic image device is intended to be a part of a continued manufacturing process utilizing continuous web approaches, it is not possible to cut out and dispose the weld seam regions.
An evident weld seam may also cause problems for the manufacturing process itself and result in a lower quality, not only influencing the region around the weld seam, but also other parts of the final product. An evident weld seam also typically results in lower production rates and influences thereby the manufacturing efficiency which eventually results in higher costs.
In the published international patent application WO 2006/056660 A1, it is disclosed to weld the edges from the inside of the cylinder to make the weld seam region appearing at the printing/embossing surface as narrow as possible. The U.S. Pat. No. 6,367,684 B1 discloses a method for smoothing and compacting of a welded joint.
For printing/embossing of most other types of products or images, based on the visual appearance of macroscopic structures, the above presented methods for making the welded joint narrow and “invisible” are typically satisfactory. However, for continuous production of synthetic image devices, a printing/embossing cylinder that has a joint which by direct visual inspection seems to be narrow and smooth, may still give rise to annoying artefacts in the final product.